Field of the Invention
The present invention relates to an image pickup apparatus which performs focus control of an image-taking optical system by using spectral information of a light from an object.
Description of the Related Art
Image pickup apparatuses such as single-lens reflex digital cameras perform focus control which detects a focus state (defocus amount) of an image-taking optical system of an interchangeable lens and moves a focus lens in the image-taking optical system by a movement amount calculated on a basis of the detected focus state to acquire an in-focus state. Many of such image pickup apparatuses introduce part of a light from an object passing through the image-taking optical system to a focus detector to perform detection of the focus state (hereinafter referred to as “focus detection”); the focus detector is provided separately from an image sensor which captures an image of the object. In these image pickup systems, the focus detector performs the focus detection, with consideration of chromatic aberration generated in the image-taking optical system, by using a specific color light and calculates the movement amount of the focus lens on the basis of the detected focus state (focus detection result). However, since a spectral sensitivity of the focus detector and that of the image sensor are different from each other, the movement amount of the focus lens calculated on the basis of the focus detection result obtained by the focus detector is not necessarily a movement amount by which a best in-focus object image can be obtained at the image sensor.
For instance, description will be made of a case, when a focal point in a red light and that in a blue light are different from each other due to the chromatic aberration generated in the image-taking optical system, the focus detector has a sensitivity to the red light as the specific color light and, on the other hand, the image sensor has an even sensitivity to the red light, a green light and the blue light. In this case, the focus detector performs the focus detection for the red light and calculates the movement amount of the focus lens by which the best in-focus image can be obtained, but a movement of the focus lens only by this movement amount is highly likely to provide blurs of the green and blue lights on the image sensor. Since visual perception of humans has a strong spectral-sensitivity characteristic, especially with respect to a green color, presence of the green light blur provides an entirely blurred image.
Japanese Patent Laid-Open No. 2006-098771 discloses an image pickup apparatus in which a first sensor having a spectral sensitivity to a visible wavelength range and a second sensor having a spectral sensitivity to an infrared wavelength range separately receive a reflected light from an object and which corrects, by using outputs from the first and second sensors, a focus detection result from a focus detector.
Similarly, Japanese Patent Laid-Open No. 2012-168429 discloses an image pickup apparatus provided with a single sensor that includes a light-receiving element (infrared receiving element) having a spectral sensitivity to an infrared wavelength range and two light-receiving elements (visible-light receiving elements) having spectral sensitivities to visible wavelength ranges different from each other. This image pickup apparatus determines a light source which illuminates an object by using output of the infrared receiving elements and output of one of the two visible-light receiving elements and further determines a color of the object by using the outputs of the two visible-light receiving elements. This image pickup apparatus corrects a focus detection result obtained by a focus detector, on a basis of the determination results.
However, the image pickup apparatus disclosed in Japanese Patent Laid-Open No. 2006-098771 does not measure a spectral intensity ratio in a visible light contained in the reflected light from the object, namely, the color of the object. For this reason, the image pickup apparatus cannot correct the focus detection result depending on the color of the object.
On the other hand, the image pickup apparatus disclosed in Japanese Patent Laid-Open No. 2012-168429 can determine, by using the single sensor that includes the infrared receiving element and the two visible-light receiving elements, both a type of the light source and the color of the object. However, the sensor only includes the single infrared receiving element and the two visible-light receiving elements, which makes it impossible to individually detect, for various objects included in an image-taking area, the type of the light source and the color of the object. Therefore, it is desirable to provide in a single sensor a large number of the infrared receiving elements and a large number of the visible-light receiving elements.
However, arranging in the single sensor such large numbers of the infrared receiving elements and the visible-light receiving elements causes the following problems. In order to prevent accuracy in the determinations of the type of the light source and the color of the object from being decreased due to appearance of a sensitivity to the infrared wavelength range in the visible-light receiving elements (in particular, light receiving elements for blue and green), it is necessary to provide, at a light-entering side further than the sensor, an infrared-cutting filter. In the case of providing the infrared-cutting filter, if the number of the infrared receiving elements and that of the visible-light receiving elements are large and a size of each of these elements is thus small, it is difficult to provide the infrared-cutting filter only to, among all the visible-light receiving elements, the light receiving elements for blue and green. On the other hand, if a large part of an infrared light is shielded by the infrared-cutting filter such that the infrared light is prevented from entering the infrared receiving elements, a sufficient output cannot be acquired from the infrared receiving elements, which makes it impossible to determine the type of the light source.